Human chorionic gonadotropin, commonly known as hCG, is a hormone found in women's blood and urine throughout pregnancy. The hCG becomes detectable in blood shortly after the time the pregnancy implants in the uterus (roughly three weeks into a four-week menstrual cycle). The hCG becomes detectable in the urine around the time of the first day of a missed menstrual period; home pregnancy tests work by detecting hCG in the urine.
In early pregnancy, the detectable levels of hCG should double roughly every two days. Slower rising or falling hCG levels can be a clue to impending miscarriage.